High-throughput technique has become an important tool in pharmaceutical and biotechnology research. Using mammalian cells to produce recombinant protein drugs has become the mainstream in current biopharmaceutical markets. Although cell lines can provide researchers with opportunities to produce a number of particular proteins, they are not always able to do so efficiently. Some cell line clones produce a protein level lower than optimum level. Other cell line clones may produce the optimal level of an expressed protein while they fail to produce entirely functional proteins due to either the formation of incorrect structure or inappropriate post-translational modification.
Though many methods have been used for sorting high expression cell lines, a number of disadvantages exist in the prior art. For example, these include the following:
(1) Single cell line isolation: For sorting high expression cell lines which largely secrete recombinant proteins, each single cell line should be isolated and cultured in an appropriate medium. The concentration of the recombinant protein in the culture medium should then be measured individually using ELISA. This method requires a huge amount of labor and is time-consuming, so cell lines that can be measured are limited.
(2) Use of a reporter gene. High-producing cell lines can be sorted by simultaneously expressing a recombinant portent of interest and a reporter gene (such as green fluorescent protein and membrane protein CD20) and measuring the expression of a reporter gene using fluorescent activated cell sorting (FACS). However, green fluorescent protein is toxic to the living cells and CD20 is only suitable for CD20-deficient cell lines. In addition, the expression level of a reporter gene in a cell is different from that of a recombinant protein of interest, so the detection of the expression level of a reporter gene cannot show the relative productivity of said recombinant protein.
(3) Low-temperature capture. A temperature as low as 4° C. is used to allow the secreted recombinant protein to transiently anchor on a cell's surface. However, this approach is only suitable for cells which secrete antibodies.
(4) Glycosylphosphatidylinositol (GPI)-anchored fusion protein. GPI anchor is attached to the C-terminal of a recombinant protein of interest so that after post translational modification, the recombinant protein is integrated into the cell membrane and can be detected by FACS. However, phosphatidylinositol-specific phospholipase C (PI-PLC) is required for cleaving the GPI-anchor to obtain a free recombinant protein.
(5) Gel microdrop technology. Protein secretory cells are encapsulated in a gel microdrop using biotinylated argarose so that the secretory recombinant proteins are retained in said gel microdrop. The high secretory cell lines could be selected by analyzing fluorescence intensity using multiple immunofluorescence labeling techniques and FACS. However, specialized instruments are required for the preparation of a gel microdrop. Moreover, since the probability of a single cell in each microdrop is very low, further multiple antibody labeling assays are required. Thus, this approach is costly and labor-intensive.
(6) Substrate-based secretion analysis. This approach is different from gel microdrop technology. The cells are modified by biotin and cultured in a high viscosity medium to accumulate the secreted recombinant protein around periplasma. Similar to the conventional technology mentioned above, multiple antibody labeling assays are required, and thus this approach is costly and labor-intensive.
Therefore, a convenient and highly efficient method for the effective selection of high-producing cell lines is still required.